As shown in FIGS. 1 and 2, a first conventional press key 10 includes a bottom board 11, a keycap 12, a linkage mechanism 13 mounted between the bottom board 11 and the keycap 12, and two springs 14. The linkage mechanism 13 has two linking members 15, each of which has two parallel supporting arms 151, 152, a connecting shaft 153 connected between the supporting arms 151, 152, two sliding rods 154 respectively formed on two ends of the connecting shaft 153 and slidably and rotatably connected to the bottom board 11, two supporting rods 155 respectively formed on outer ends of the supporting arms 151, 152 and rotatably connected to the keycap 12, a one-tooth piece 156 formed on one of the supporting arms 151, 152, and a two-teeth piece 157 formed on the other one of the supporting arms 151, 152. The one-tooth piece 156 of one of the linking members 15 engages the two-teeth piece 157 of the other one of the linkage members 15 to form a gear hinge 158. The two-teeth piece 157 of the one of the linking members 15 engages the one-tooth piece 156 of the other one of the linkage members 15 to form another gear hinge 158 (only one gear hinge 158 is visible in FIG. 2). Each of the springs 14 is mounted between the bottom board 11 and a respective one of the connecting shafts 153.
Referring to FIGS. 3 and 4, a second conventional press key 20 includes a bottom board 21, a keycap 22, and a linkage mechanism 23. The bottom board 21 has two receiving units 211, each of which has an inner sliding groove 2111, and two outer sliding grooves 2112. The linkage mechanism 23 has two arm members 24, each of which has a first arm portion 241, a second arm portion 242 parallel to the first arm portion 241, a receiving cavity 243 formed in the first arm portion 241, an abutting portion 244 formed on the first arm portion 241, an inner supporting rod 245 and a first outer supporting rod 246 formed on the first arm portion 241, and a protruding portion 247, a wing portion 248 and a second outer supporting rod 249 formed on the second arm portion 242. For one of the arm members 24, the inner supporting rod 245 slidably and rotatably engages the inner sliding groove 2111 of one of the receiving units 211, the first outer supporting rod 246 slidably and rotatably engages one of the outer sliding grooves 2112 of the one of the receiving units 211, and the second outer supporting rod 249 slidably and rotatably engages one of the outer sliding grooves 2112 of the other one of the receiving units 211. For the other one of the arm members 24, the inner supporting rod 245 slidably and rotatably engages the inner sliding groove 2111 of the other one of the receiving units 211, the first outer supporting rod 246 slidably and rotatably engages the other one of the outer sliding grooves 2112 of the other one of the receiving units 211, and the second outer supporting rod 249 slidably and rotatably engages the other one of the outer sliding grooves 2112 of the one of the receiving units 211. Each of the arm members 24 further has a connecting shaft 240 connected between the first arm portion 241 and the second arm portion 242, and slidably and rotatably connected to the keycap 22. The protruding portion 247 of each of the arm members 24 is received in the receiving cavity 243 of the other one of the arm members 24, and abuts against the abutting portion 244 of the other one of the arm members 24, such that the protruding portion 247 and the wing portion 248 of one of the arm members 24 cooperate with the receiving cavity 243 and the abutting portion 244 of the other one of the arm members 24 to forma gear hinge 230, and that the protruding portion 247 and the wing portion 248 of the other one of the arm members 24 cooperate with the receiving cavity 243 and the abutting portion 244 of the one of the arm members 24 to form another gear hinge 230.
However, due to the fact that, for each gear hinge 158 of the first conventional press key 10, the engagement between the one-tooth piece 156 and the two-teeth piece 157 relies merely on the abutment of the one-tooth piece 156 against the two-teeth piece 157, and that, for each gear hinge 230 of the second conventional press key 20, the connection between the first arm portion 241 and the second arm portion 242 relies merely on the abutment of the protruding portion 247 against the abutting portion 244, the linkage mechanism 13 of the first conventional press key 10 and the linkage mechanism 23 of the second conventional press key 20 cannot be pre-assembled, thereby resulting in relatively low assembly efficiencies of the first conventional press key 10 and the second conventional press key 20. Besides, the structural strengths of the gear hinges 158 of the first conventional press key 10 and the gear hinges 230 of the second conventional press key 20 are not great, thereby resulting in relatively high damage rates of the first conventional press key 10 and the second conventional press key 20. Furthermore, the structure of the bottom board 21 of the second conventional press key 20 is complicated and thus requires relatively high manufacturing costs.